1. Technical Field
The present invention relates to a capacitor unit and more particularly, to a capacitor unit and a method of forming the same.
2. Discussion of the Related Art
The size of digital circuits incorporated into systems-on-chip (SOCs) has rapidly decreased; however, the size of analog circuits has not decreased very much. This is in large part attributable to the inability to substantially reduce the size of passive elements such as capacitors. Thus reducing the size of capacitors is important to achieve a high degree of integration in a semiconductor device.
The capacitance of a capacitor is proportional to a dielectric constant (k) and an area of a dielectric layer. Accordingly, when designing a capacitor with a desired capacitance, a dielectric material having a high k may be used to produce a smaller capacitor.
However, when the high-k dielectric material is used for the dielectric layer, a voltage coefficient of capacitance (VCC) may increase.
Referring to FIGS. 1A and 1B, when a first dielectric layer is formed using a high-k dielectric material such as hafnium oxide, hafnium carbon oxynitride and hafnium oxide (see FIG. 1A), a normalized capacitance of a capacitor having the first dielectric layer is more widely varied than that of a capacitor having a second dielectric layer including a low-k dielectric material such as silicon nitride, as the voltage applied to the capacitor is increased.
Particularly, when a capacitor has a polysilicon-insulator-polysilicon (PIP) structure, impurities are implanted into the polysilicon layer in order to decrease a VCC of the capacitor. Additionally, even the polysilicon layer doped with the impurities has a depletion area, so that the VCC may not be sufficiently decreased. Thus, the capacitor having the PIP structure is not suitable for an analog capacitor, which is employed in an analog-to-digital converter (ADC), a radio frequency (RF) device, a complementary metal-oxide semiconductor (CMOS) image sensor (CIS), etc. Accordingly, a capacitor having a metal-insulator-metal (MIM) structure has been used.
The capacitor having the MIM structure does not have a depletion area, and therefore the capacitor has a VCC smaller than that of the capacitor having the PIP structure. However, even the capacitor having the MIM structure does not have a sufficiently small VCC, and reducing the VCC may be needed in order to serve as an analog capacitor having high precision.
A capacitance of a capacitor and a voltage applied to the capacitor may be represented by Equation 1.C=C0(aV2+bV+1)  [Equation 1]
Here, C refers to a capacitance of a capacitor, and C0 indicates the capacitance of the capacitor when a voltage has not been applied thereto.
Referring to Equation 1, the VCC depends on the first coefficient “a” related to the square of a voltage V and the second coefficient “b” related to the voltage V. Thus, the first and second coefficients are decreased in order to decrease the VCC.